Band pass amplifier coupling



April 14, 1936. R BRADEN 2,037,61

BAND PASS AMPLIFIER COUPLING Filed Dec. 12, 1929 Z a Sheets-Sheet 1 1 19I 1 7 1F fi 6 f 07 7 mum ATTORNEY -April 14; 1936. R BRADEN 2,037,614

' BAND PASS AMPLIFIER COUPLING Filed Dec. 12, 1929 5 sheets-sheet :5

' a l I' I I I l I I I I l 51 0 500 700 800 .900 /M //00 L 00 300 #00 AWM70 Flifll/iA/CY W KC- RENE A. BR DEN BY ATTORNEY Patented Apr. 14, 1936UNITED STATES PATENT OFFICE BAND PASS AMPLIFIER COUPLING Rene A. Braden,New York, N. Y., assignor to Radio Corporation of America, a corporationof Delaware My present invention relates to amplifiers, and moreparticularly, to a band pass amplifier adapted to operate with aconstant accepted band width.

In a co-pending application Serial No. 278,105 filed May 16, 1928, Ihave disclosed band pass amplifiers of the type comprising two or morecircuits, each tuned to the mid-band frequency, and coupling means forthe circuits arranged so as to produce a constant accepted band width.It was further shown, in the said application, that to produce thislatter condition the coupling between the tuned circuits must vary asthe circuits are tuned to various wave frequencies, the variation beingsuch that the product of coupling and wave frequency is approximatelyconstant. Coupling means of various types were, also, disclosed in thesaid application whereby the accepted band width was maintained at aconstant value as the circuits were tuned to receive at various wavefrequencies.

Now, I have discovered, by further experimentation, additionalarrangements for coupling tuned circuits in a band pass amplifier of thetype described in the above mentioned application.

In the new method two independent couplings are provided,'only one ofthese couplings being variable simultaneously with the tuning control,and being mechanically linked to the latter. Thus, as the circuits aretuned for various wave frequencies, the effective coupling is caused tovary in the proper manner to maintain constant accepted band width.

An object of the invention is to provide a method of maintainingconstant accepted band width in a band pass amplifier which consists inproviding two opposing couplings, and varying one couplingsimultaneously with the tuning of the tuned circuits.

Still another object of the invention is to provide a band passamplifier comprising a vacuum tube, a tuned circuit in the outputcircuit of the tube, a second vacuum tube, a tuned circuit in the inputcircuit of the second tube, capacitative coupling between the tunedcircuits, inductive coupling reversed in phase with respect to thecapacity coupling, means for varying either capacity or inductivecoupling, or both, and means for causing the variable coupling orcouplings to vary as the tuned circuits are adjusted to receive atvarious wave frequencies.

Other objects of ,the invention are to improve generally the simplicityand efiiciency of band pass amplifier coupling arrangements, and toprovide devices of the latter type which are not only reliable inoperation, but economical and simple to manufacture.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims, the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawings in which I have indicateddiagrammatically several circuit organizations whereby my invention maybe carried. into effect.

In the drawings:

Fig. 1 shows diagrammatically a novel type of coupling arrangement,

Figs. 2 to 5 inclusive show modified coupling arrangements embodying thepresent invention,

Figs. 6 and 7 are graphic representations of the coupling arrangements.

Referring, now, to the accompanying drawings, in which like charactersof reference indicate the same elements in the different modifications,the numeral l indicates an electron discharge tube of either the threeelectrode or four electrode type, and shown preferably as the latter,the incoming signal energy to be amplified being impressed between thegrid and cathode of the tube, and designated as input. The referencenumerals 3 and 4 denote an inductance and condenser respectively thusforming the primary tuned circuit which is tuned to the midbandfrequency of the incoming signal.

The numerals 6 and I represent a condenser andinductance respectively,and form the secondary tuned circuit, which is also tuned to themid-band frequency. The numeral 8 designates a second electron dischargedevice, preferably of the four electrode type, whose grid is connectedto receive the voltage which is developed across the secondary circuit6, 'l. The term output, in each modification, represents the circuit towhich any subsequent circuits are connected to employ the amplifiedoutput of the device 8.

It is to be understood that the circuits embodying my invention areintended to select a definite band of frequencies from any part of arelatively wide range of frequencies. For example, a broadcast receiverembodying the invention would be capable of selecting and amplifying allsignal currents contained within a band 10 kilocycles in width and wouldbe able to select this 10 kilocycle band from any part of the frequencyspectrum between 550 kilocycles and 1500 kilocycles. The mid-bandfrequency in this case is equal to the carrier wave frequency, and theaccepted band extends equally on each side of the carrier frequency,

The novel method of a coupling as referred to heretofore is shown inFig. 1, the primary tuned circuit 3, 4 being coupled to the secondarytuned circuit, 6, 1, l3, by mutual inductance of magnitude M, betweenthe coils l3 and 3, and also by the condenser 19 of capacity C.

Either C or M must be varied to keep the band width constant, and it isintended that the coupling control be combined in any suitable mannerwith the tuning control. The advantage of this circuit is illustrated bythe curves in Figs. 6 and 7.

Fig. 6 shows the coupling capacity C required for a constant band widthof 10 kilocycles, when M/L has various values. (L is the inductance ofcoil 3, and of coils l and 13 in series.) The value of L assumed incalculating these curves was 253 microhenries. If the curve for M/L=0 iscompared with that for M/L:.02, it will be seen that in the latter casethe required range of adjustment of C is much smaller than in the formercase.

Referring to Fig. 7 which shows the values of M required when G isconstant, it is seen that the variation of M is much smaller when C=.50mmf., than when C=0. It should be noted that the coil I3 must beconnected so as to reverse the inductive coupling with respect to thecondenser coupling, this being the condi tion described in co-pendingapplication Serial No. 278,105, filed May 16, 1928 which causes the bandwidth to remain almost constant. Then, only a relatively small variationof one of the two couplings is required to maintain absolute constancyof the band width.

In Fig. 2 is shown a modified circuit in which the coupling coil I3 isreplaced by the link coupling coils 24 and 25 and the variableinductance 25 having an inductance L1. The inductance 26 controls theelfective inductive coupling, and otherwise the operation of the circuitis the same as that of Fig. 1.

Fig. 3 shows another modification in which the inductive coupling iscontrolled by movement of the coils 2d and 25 so as to vary the mutualinductances M1 and M2.

In Figs. 1, 2, and 3 I have not shown mechanical linking between thecoupling adjustment and the tuning adjustment because in these threefigures two alternative coupling adjustments are shown, e. g., in Fig. 1either the coupling condenser !9 or the coupling coil 13 should belinked mechanically to the tuning control, but it is not intended thatboth of these shall be so linked.

Fig. 4 shows still another modification in which inductive couplingoccurs in the form of mutual inductance of magnitude M between the coils3 and l, and the capacity coupling is provided by the condenser 9, 50,H.

Fig. 4 shows a circuit employing a novel type of variable couplingcondenser, one form of which has been described in my copendingapplication, Serial No. 278,105, filed May 16th, 1928. In this circuitthe coil 3 and the Variable condenser 4 constitute the primary circuit,the coil 1 and variable condenser G the secondary circuit, and theprimary and secondary circuits are coupled by the electrostatic capacitybetween the plates 9 and H! which are attached to the high potentialends of the condensers 4 and 6, respectively. A metal plate I l isconnected to ground (i. e. to the filament side of the circuit) and ismounted so that it can be slid between the plates 9 and 10 or drawn out.It must be insulated from the plates 9 and It and should move in a planeparallel to them and half way between. The capacity between 9 and I0 isreduced by the plate II, the amount of reduction being roughlyproportional to the area of H which is between the plates 3 and i0. Bythis means the coupling capacity can be reduced from a comparativelylarge value almost to zero, and the ratio of maximum to minium capacityis much greater than can be secured with the conventional two-platevariable condenser of equal maximum capacity. Instead of being mountedas indicated in the drawings, the plate ll may be mounted so as torotate on the shaft which carries the rotor plates of the variablecondensers 5 and 6, and the plates 9 and 10 could then be convenientlyattached to and supported by the stator units of the condensers 4 and 6.

A modification is shown in Fig. 5, in which the inductive coupling isdirectly between the coils 3 and l, and the. coupling condenser 19 isconnected in series with coils 24 and 25, the latter being coupledinductively to coils 3 and 1, respectively.

In Figs. 1 to '7 inclusive combined inductive and capacitative couplingsare utilized, the inductive coupling being reversed in phase withrespect to the condensive coupling. That is, the

coupling network is arranged so that the voltage I induced in 'thesecondary tuned circuit by the inductive coupling is opposite indirection to the voltage produced therein by the condensive coupling.Thus, at low wave frequencies, where the condensive coupling isineffective, the inductive coupling provides sufficient coupling;whereas at higher wave frequencies, the inductive coupling is too large,but the condensive coup-ling becomes increasingly effective. As statedheretofore, this has been clearly described by me in my co -pendingapplication Serial No. 278,105.

It is pointed out, however, that by utilizing the present inventionconsisting in varying one of the couplings at a predetermined rate withthe tuning adjustment, the effective coupling between the coup-led tunedcircuits is maintained such that the accepted band Width is keptabsolutely constant throughout the tuning range of the coupled circuits.Figs. 6 and 7 clearly point out the rates at which the capacitycoupling, or the inductive coupling, is to be varied in a combinedcapacitative-inductive coupling band pass network to achieve absoluteconstancy of accepted band width throughout the tuning range of thecoupled circuits.

While I have indicated and described several systems for carrying myinvention into effect it will be apparent to one skilled in the art thatmy invention is by no means limited to the particular organizationsshown and described, but that many modifications in the circuitarrangements, as well as in the apparatus employed, may be made withoutdeparting from the scope of my invention as set forth in the appendedclaims.

What I claim is:

1. A band pass coupling network comprising two tuned circuits, combinedcapacitative and inductive couplings between the circuits, and means forvarying at least one of said couplings simultaneously with tuningadjustments at a predetermined rate so as to maintain the accepted bandwidth absolutely constant, throughout a frequency range of 550 to 1500kilocycles, one of said couplings being reversed in phase with respectto the other.

2. A band pass amplifier coupling network comprising a pair of tunedcircuits, combined capacitative and inductive couplings between thecircuits, the couplings being so related that the voltages induced bythem in the secondary tuned circuit are in opposite directions, andmeans including an adjustable reactance for varying at least one of saidcouplings simultaneously with tuning adjustments at a rate so as tomaintain the accepted band width absolutely constant over the amplifiertuning range.

3. A band pass coupling network comprising at least two tuned circuits,combined capacitative and. inductive couplings between the circuits, thecouplings being so related that the voltages induced by them in thesecondary tuned circuit are in opposite directions, and means includingan adjustable reactance common to the tuned circuits for varying atleast one of said couplings simultaneously with tuning adjustments at arate so as to maintain the accepted band width absolutely constantthrough a tuning range of 550 to 1500 kilocycles.

4. A band pass amplifier coupling network comprising at least two tunedcircuits, combined capacitative and inductive couplings between thecircuits, the couplings being so related that the voltages induced bythem in the secondary tuned circuit are in opposite directions, andmeans including an adjustable condenser mechanically linked with thecircuit tuning means for varying at least one of said couplingssimultaneously with tuning adjustments at a rate so as to maintain theaccepted band width absolutely constant throughout the amplifier tuningrange.

5. A band pass amplifier coupling network comprising at least two tunedcircuits, combined capacitative and inductive couplings between thecircuits, said couplings being relatively reversed, and means includinga link circuit having an adjustable reactance for varying at least oneof said couplings simultaneously with tuning adjustments at a rate so asto maintain the accepted band width absolutely constant through theamplifier tuning range.

6. A band pass amplifier comprising a vacuum tube, 'a tuned circuit inthe output circuit of the tube, a second vacuum tube, a tuned circuit inthe input circuit of the second tube, capacity coupling between thetuned circuits, inductive coupling between the tuned circuits reversedin phase with respect to the capacity coupling, and means for causingone of said couplings to vary at a predetermined rate as the tunedcircuits are adjusted to receive various wave frequencies, said rate ofcoupling variation being such that the accepted band width is maintainedexactly constant throughout the tuning range of the amplifier.

'7. An amplifier adapted to operate through the broadcast rangecomprising a pair of oscillation circuits, an electron discharge tubehaving one of said circuits connected between its input electrodes,capacity coupling between the circuits, inductive coupling between thecircuits reversed with respect to the capacity coupling, means fortuning said circuits to a desired frequency, and means including anadjustable grounded plate for vary ing said capacity couplingsimultaneously with operation of the tuning means, at such a rate thatsaid amplifier is uniformly selective through said range.

8. An amplifier adapted to operate over the broadcast range comprising apair of oscillation circuits, ,an' electron discharge tube having one ofsaid circuits connected between its input electrodes, capacity couplingbetween the high potential points of said circuits, inductive couplingbetween the circuits reversed in phase with respect to the capacitycoupling, means for tuning said circuits to a desired frequency, andmeans for varying said inductive coupling simultaneously with operationof the tuning means at a rate such that the amplifier is uniformlyselective over the said range.

9. An amplifier adapted to operate over the broadcast range comprising apair of oscillation circuits, an electron discharge tube of the screengrid type having one of said circuits connedted between its inputelectrodes, capacity coupling between the circuits, inductive couplingbetween the circuits reversed in phase with respect to the capacitycoupling, variable condensers for tuning said circuits to a desiredfrequency, and means for varying said inductive coupling simultaneouslywith operation of the tuning condensers at such a rate that saidamplifier is uniformly selective through said range.

10. An amplifier adapted to operate'over the broadcast range comprisinga pair of oscillation circuits, an electron discharge tube having one ofsaid circuits connected between its input electrodes, capacity couplingbetween the circuits, inductive coupling between the circuits, thecouplings being so related that the voltages induced by them in thesecondary tuned circuit are in opposite directions, means for tuningsaid circuits to a desired frequency, and means for varying theinductive coupling simultaneously with operation of the tuning means ata rate such that the amplifier is uniformly selective over the saidrange.

11. An amplifier adapted to operate over the broadcast range comprisinga pair of oscillation circuits, an electron discharge tube having one ofsaid circuits connected between its input electrodes, capacity couplingbetween the circuits, a link circuit including inductive couplingbetween the circuits, the couplings being so related that the voltagesinduced by them in the secondary tuned circuit are in oppositedirections, means for tuning said circuits to a desired frequency, andmeans for Varying one of said couplings simultaneously with operation ofthe tuning means at such a rate that said amplifier is uniformlyselective over said range.

12. An amplifier operating over the broadcast range comprising a pair ofoscillation circuits, an electron discharge tube having one of saidcircuits connected between its output electrodes, capacity couplingbetween the high potential points of said circuits, inductive couplingbetween the circuits reversed in phase with respect to the capacitycoupling, variable condensers for tuning said circuits to a desiredfrequency, and means including an adjustable inductor for varying saidinductive coupling simultaneously with operation of the tuning means ata predetermined rate such that the amplifier is uniformly selective overthe said range.

13. A band pass amplifier comprising a pair of oscillation circuits, ascreen grid tube having one of said circuits connected between its inputelectrodes, capacity coupling between the high potential points of saidcircuits, a link circuit including inductive coupling between thecircuits, the inductive coupling being reversed in phase with respect tothe capacity coupling, means for tuning said circuits to a desiredmid-band frequency of the broadcast range, and a single means forvarying one of said couplings simultaneously with operation of thetuning means at such a rate that the accepted band width is exactlyconstant throughout said range.

14. The method of transferring electrical energy having a predeterminedfrequency band width throughout a predetermined range of frequenciesfrom an exciting circuit to a tunable absorbing circuit, which consistsof transferring the energy both electromagnetically andelectrostatically in separate paths, and causing said electromagnetictransfer to vary at such a rate as said absorbing circuit is tunedthrough said range that said band width is maintained absolutelyconstant throughout said range.

15. The method of transferring electrical energy having a band width often kilocycles through a range of frequencies between 550 and 1500kilocycles from an exciting circuit to an absorbing circuit, whichconsists of transferring energy both electromagnetically andelectrostatically in separate paths, adjusting said absorbing circuitfor resonance with desired current frequencies in said exciting circuit,and adjusting the rate of variation of the electromagnetic energytransfer as said absorbing circuit is adjusted through said frequencyrange to maintain said band width ten kilocycles wide throughout saidfrequency range.

16. An electrical system comprising an exciting circuit and a tunableabsorbing circuit, an electrostatic coupling between said circuitsadapted to decrease in a predetermined way with increase of frequency assaid absorbing circuit is tuned, and an electromagnetic coupling betweensaid circuits so related to the first coupling as to transfer energy tothe absorbing circuit reversed in phase with respect to the energytransferred by said electrostatic coupling, and additional means forvarying said electromagnetic coupling simultaneously with tuning of saidabsorbing circuit, the rate of variation of said electromagneticcoupling being so chosen as to render said absorbing circuit uniformlyselective throughout its tuning range.

1'7. The method of transferring electrical energy having a predeterminedfrequency band width throughout a range of frequencies from an excitingcircuit to a tunable absorbing circuit, which consists of transferringthe energy both electromagnetically and electrostatically in separatepaths, and causing said electromagnetic energy transfer to vary as saidabsorbing circuit is tuned in such sense as to maintain said band widthabsolutely constant throughout said range of frequencies.

18. An electrical system comprising an exciting circuit and a tunableabsorbing circuit, an electrostatic coupling between said circuits, anelectromagnetic coupling between said circuits, said couplings being sopoled that their combined energy transfer remains approximately constantwith frequency as said absorbing circuit is variably tuned through apredetermined frequency range, and additional means, simultaneouslyoperative with the tuning of the absorbing circuit, for adjusting theelectromagnetic coupling to maintain the combined energy transferabsolutely constant throughout said frequency range.

19. A band pass amplifier coupling network comprising a pair of tunedcircuits, combined capacitative and inductive couplings between thecircuits, the couplings being so related that the voltages induced bythem in the secondary tuned circuit are in opposite directions, andmeans including an adjustable reactance for varying at least one of saidcouplings simultaneously with tuning adjustments at a predetermined ratewhereby a precisely determined band width is transmitted over theamplifier tuning range.

20. A band pass amplifier comprising a vacuum tube, a tuned circuit inthe output circuit of the tube, a second vacuum tube, a tuned circuit inthe input circuit of the second tube, capacity coupling between thetuned circuits, inductive coupling between the tuned circuits reversedin phase with respect to the capacity coupling, and means for causingone of said couplings to vary at a predetermined rate as the tunedcircuits are adjusted to receive various wave frequencies, said rate ofcoupling variation being such that a precisely determined band width istransmitted throughout the tuning range of the amplifier.

21. An amplifier adapted to operate over an extended tuning rangecomprising a pair of oscillation circuits, an electron discharge tubehaving one of said circuits connected between its input electrodes,capacity coupling between the circuits, inductive coupling between thecircuits reversed in phase with respect to the capacity coupling,variable condensers for tuning said circuits to a desired frequency, andmeans for varying said inductive coupling simultaneously with operationof the tuning condensers at such a rate that said amplifier is uniformlyselective through said range, said variable inductive coupling attaininga minimum value at some point within said tuning range.

22. An amplifier adapted to operate over an extended tuning rangecomprising a pair of oscillation circuits, an electron discharge tubehaving one of said circuits connected between its input electrodes,capacity coupling between the circuits, inductive coupling between thecircuits reversed in phase with respect to the capacity coupling,variable condensers for tuning said circuits to a desired frequency, andmeans for varying said capacity coupling simultaneously with operationof the tuning condensers at such a rate that said amplifier is uniformlyselective through said range, said variable capacity coupling attaininga maximum value at some point within said tuning range.

RENE A. BRADEN.

